1. Field of Invention
The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to a safety vent valve. Yet more specifically, the present invention relates to a safety vent valve for a perforating gun system.
2. Description of Prior Art
Perforating systems are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore. The casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.
One typical example of a perforating system 4 is shown in FIG. 1. As shown, the perforating system 4 comprises one or more perforating guns 6 strung together to form a perforating gun string 3, these strings of guns can sometimes surpass a thousand feet of perforating length. Connector subs 18 provide connectivity between each adjacent gun 6 of the string 3. Many gun systems, especially those comprised of long strings of individual guns, are conveyed via tubing 5. Others may be deployed suspended on wireline or slickline (not shown).
Included with the perforating gun 6 are shaped charges 8 that typically include a housing, a liner, and a quantity of high explosive inserted between the liner and the housing. When the high explosive is detonated, quickly expanding explosive gases are formed whose force collapses the liner and ejects it from one end of the charge 8 at very high velocity in a pattern called a “jet” 12. The jet 12 perforates the casing and the cement and creates a perforation 10 that extends into the surrounding formation 2. The resulting perforation 10 provides fluid communication between the formation 2 and the inside of the wellbore 1. In an under balanced situation (where the formation pressure exceeds the wellbore pressure) formation fluids flow from the formation 2 into the wellbore 1, thereby increasing the pressure of the wellbore 1. Moreover, as the explosive gases cool and contract, a large pressure gradient is created between the inside of the perforating gun body 14 and the wellbore 1. This pressure differential in turn draws wellbore fluid within the perforating gun body 14 through gun apertures 16.
FIGS. 2a and 2b illustrate a portion of a gun string 3 for providing additional detail of the connector sub 18 disposed between the two perforating guns 6. As shown, the connector sub 18 has a protruding member 19 on each of its ends formed to mate with a corresponding recess 21 provided on the end of each perforating gun 6. The guns 6 as shown are secured to the connector sub 18 by a series of threads 23 formed on the inner diameter of the recesses 21 and the outer diameter of the protruding member 19.
Also disposed within the gun string is a detonating cord 20 for providing an initiating/detonating means for the shaped charge 8. Detonation of the shaped charge 8 is accomplished by activating the detonating cord 20 that in turn produces a percussive shockwave for commencing detonation of the shaped charge explosive 8. Typically the shockwave is initiated in the detonating cord 20 at its top end (i.e. closest to the surface 9) and travels downward through the gun string 3. To ensure propagation of the shockwave to each individual gun 6 making up the gun string 3, each connecting sub 18 is also equipped with a section of detonating cord 20. The section of detonating cord 20 in the connecting sub 18 resides in a cavity 22 formed therein. Transfer charges 24 on the end of each segment of the detonating cord 20 continue travel of the shock wave from the end of one gun body 6, to the section of detonating cord 20 in the connecting sub 18, from the connecting sub 18 to the next adjacent gun body 6, and so on. The shock wave transfer function of the transfer charges 24 produces a passage 26 between the gun bodies 6 and the connecting sub 18. As shown in FIG. 2b, the shaped charge 8 detonates in response to exposure of the shock wave produced by the detonating cord 20. Detonation of the shaped charge 8 in turn leaves an aperture 16 that provides fluid flow from the wellbore 1 to inside of the gun body 14. Similarly, detonation of the transfer charges 24 in response to the detonating cord shock wave, creates the passage 26 provides a fluid flow conduit between the inside of the perforating gun bodies 6 and the connecting sub cavity 22. Accordingly, the cavity 22 is subject to wellbore pressures subsequent to exposure of the detonating cord shock wave. Often the debris within the wellbore fluid can be carried with the fluid into the cavity 22. When retrieving the gun system 4 from the wellbore 1, the cavities 22 will be vertically oriented that in turn can allow the fluid debris to collect within the passages 26 thereby creating a potential clogging situation that can trap the wellbore fluid within the connecting sub 18. Since the wellbore fluid pressure can often exceed 1000 psi, this trapped pressure can present a personnel hazard during disassembly of the gun string 3. Therefore, an apparatus and method for eliminating the potential for trapped pressure within the connecting sub 18 is needed.
Perforating gun strings are typically assembled at a manufacturing facility then shipped to the job site. Sometimes the assembled gun strings are stored before use at the manufacturing facility, at an intermediate location during shipping, or at the job site. The explosives used in the shaped charges are reactive at high temperatures and may begin to expel gasses when heated. The gun body may become excessively heated when exposed to fire, prolonged direct sunlight, as well as other heat sources. This off gas situation may occur for temperatures as low as 400° F. Since the gun bodies are pressure sealed to prevent inflow of wellbore fluids, explosive off gassing due to heating can increase gun body pressure past its burst pressure. Accordingly a need exists to maintain gun body pressure below its burst pressure.